Tissue ablation may be used to treat a variety of clinical disorders. For example, tissue ablation may be used to treat cardiac arrhythmias by destroying (e.g., at least partially or completely ablating, interrupting, inhibiting, terminating conduction of, otherwise affecting, etc.) aberrant pathways that would otherwise conduct abnormal electrical signals to the heart muscle. Several ablation techniques have been developed, including cryoablation, microwave ablation, radio frequency (RF) ablation, and high frequency ultrasound ablation. For cardiac applications, such techniques are typically performed by a clinician who introduces a catheter having an ablative tip to the endocardium via the venous vasculature, positions the ablative tip adjacent to what the clinician believes to be an appropriate region of the endocardium based on tactile feedback, mapping electrocardiogram (ECG) signals, anatomy, and/or fluoroscopic imaging, actuates flow of an irrigant to cool the surface of the selected region, and then actuates the ablative tip for a period of time and at a power believed sufficient to destroy tissue in the selected region.
Although commercially available ablative tips may include thermocouples and/or other sensors (such as thermistors, other conventional temperature-measurement devices, e.g., devices that merely detect or measure a temperature at or near the temperature measure device, etc.) for providing temperature feedback via a digital display, such thermocouples typically do not provide meaningful temperature feedback during irrigated ablation. For example, the thermocouple or other sensor only measures surface temperature, whereas the heating or cooling of the tissue that results in tissue ablation may occur at some depth below the tissue surface. Moreover, for procedures in which the surface of the tissue is cooled with an irrigant, the thermocouple will measure the temperature of the irrigant, thus further obscuring any useful information about the temperature of the tissue, particularly at depth. As such, the clinician has no useful feedback regarding the temperature of the tissue as it is being ablated or whether the time period of the ablation is sufficient. Because the clinician lacks such information, the clinician furthermore cannot regulate the power of the ablation energy so as to heat or cool the tissue to the desired temperature for a sufficient period of time.
Accordingly, it may only be revealed after the procedure is completed—for example, if the patient continues to experience cardiac arrhythmias—that the targeted aberrant pathway was not adequately interrupted. In such a circumstance, the clinician may not know whether the procedure failed because the incorrect region of tissue was ablated, because the ablative tip was not actuated for a sufficient period of time to destroy the aberrant pathway, because the ablative tip was not touching or sufficiently touching the tissue, because the power of the ablative energy was insufficient, or some combination of the above. Upon repeating the ablation procedure so as to again attempt to treat the arrhythmia, the clinician may have as little feedback as during the first procedure, and thus potentially may again fail to destroy the aberrant pathway. Additionally, there may be some risk that the clinician would re-treat a previously ablated region of the endocardium and not only ablate the conduction pathway, but damage adjacent tissues.
In some circumstances, to avoid having to repeat the ablation procedure as such, the clinician may ablate a series of regions of the endocardium along which the aberrant pathway is believed to lie, so as to improve the chance of interrupting conduction along that pathway. However, there is again insufficient feedback to assist the clinician in determining whether any of those ablated regions are sufficiently destroyed.
Despite the promise of precise temperature measurement sensitivity and control offered by the use of radiometry, there have been few successful commercial medical applications of this technology. One drawback of previously-known systems has been an inability to obtain highly reproducible results due to slight variations in the construction of the microwave antenna used in the radiometer, which can lead to significant differences in measured temperature from one catheter to another. Problems also have arisen with respect to orienting the radiometer antenna on the catheter to adequately capture the radiant energy emitted by the tissue, and with respect to shielding high frequency microwave components in the surgical environment so as to prevent interference between the radiometer components and other devices in the surgical field.
Radiofrequency ablation techniques have developed a substantial following in the medical community, even though such systems can have severe limitations, such as the inability to accurately measure tissue temperature at depth, e.g., where irrigation is employed. However, the widespread acceptance of RF ablation systems, extensive knowledge base of the medical community with such systems, and the significant cost required to changeover to, and train for, newer technologies has dramatically retarded the widespread adoption of radiometry.